An important class of photographic films are black-and-white silver halide films intended to be used for contact exposures in the field of graphic arts. Such films require a high degree of dimensional stability as well as a surface which is non-tacky, and has a suitable degree of roughness to facilitate rapid vacuum draw-down during contact exposure. Advantageously, these films are relatively low in photographic speed so that they can be used under bright safelight or even ordinary room-light conditions. To facilitate handling in use, it is highly desirable that the front and back surfaces of the contact film be readily distinguishable by the user.
An additional highly desirable property for the aforesaid graphic arts contact films is the ability to optically spread the image during contact exposure. Thus, for example, in employing dot image originals, it is desirable to be able to regulate the degree of dot growth that occurs on contact exposure. Also, the original which is to be exposed in the contact exposure process can, in some instances, be a multi-layer original, that is, an original in which two or more elements have been stacked together as an assembly. Such a multi-layer assembly can include both line image originals and dot image originals. The ability to optically spread the image during exposure is critically important in handling such stacked originals in a single exposure process. In particular, optical spreading can improve the image quality of characters produced from originals which are out of contact with the contact film.
Optical image spread is distinguished from chemical image spread which involves infectious imagewise development of unexposed photographic silver halide grains in close proximity to exposed photographic silver halide grains. Chemical image spread is achieved by incorporation of a nucleating agent in either the film or the developing solution. A given system can employ either optical image spread or chemical image spread or both.
To facilitate optical spreading of an image during contact exposure, it is important that the silver halide emulsion layer of the contact film be widely spaced from the surface of the contact film that comes into face-to-face contact with the original, such surface typically being the surface of a protective overcoat layer which serves as the outermost layer of the film.
It is increasingly common in the graphic arts to employ a computer assisted "dry dot etching" process in making color corrections to halftone separations. The process can be carried out without the use of masks for color correction of an entire separation, or it can be done with hand-cut masks or photographic masks for local color corrections. Techniques used in dry dot etching vary from shop to shop, but all depend on the ability of a contact or duplicating film to change dot size with overexposure (overexposure meaning an exposure greater than that necessary to produce dot-for-dot reproduction in the midtone dot values). Generally, the dot-change exposure technique starts with a dot-for-dot exposure and adds a "bump", or additional, exposure to produce the desired change in dot value. This "bump" exposure may be confined to a localized area of the subject by masking, or it may be combined with the dot-for-dot exposure to make an overall change to the separation. Contact films which are best suited for use in dry dot etching are those that provide a high degree of optical spread.
It is exceedingly difficult to incorporate in a photographic film all of the properties that are desirable for use as a contact exposure film in the graphic arts. Thus, for example, the use of a very thick overcoat layer to provide the desired spacing between the surface of the film and the silver halide emulsion layer is impractical, since it adversely affects dimensional stability. To improve dimensional stability, a polymer latex can be incorporated in the overcoat layer, but this tends to render the surface undesirably tacky. Also, if the overall thickness of the hydrophilic layers, including the emulsion layer and overcoat layer, becomes too great, the diffusion of developing agents and fixing agents to the emulsion layer will be impeded and the time required for development and fixing will be excessive. Yet another problem will be the prolonged drying period needed to dry the processed film.
The present invention is directed to a novel contact film for use in the graphic arts which effectively overcomes all of the above problems and combines a wide variety of desired features in a single film.